Polycarboxylic acids have long been known to be useful, usually in the salt form, as detergent builders or sequestrants. Also, ether carboxylates useful as metal sequestering and detergent builders have been known and are most desirable for their desirable laundering applications.
Because these ether carboxylates have such effective sequestering ability they have become attractive in recent times for the replacement of sodium tripolyphosphate which has long been the leading detergent builder or sequesterant. Examples of prior art efforts to provide ether carboxylates detergent builders or sequesterants are found in U.S. Pat. Nos. 3,635,830; 3,692,685 which relate to the use of oxydisuccinic acid salts as detergent builders. Another example of the prior art employing a carboxylate ether is found in U.S. Pat. No. 3,914,927 relating to carboxymethyloxysuccinates.
While these compounds in the prior art have utility as a builder or sequesterant in laundry detergent formulations it has been found that mixtures of certain low molecular weight ether carboxylates are more attractive and cost effective for such utility. In the field of detergent builders and sequesterants for laundry detergent formulations low cost of the components is extremely important because it is a very competitive market. While many ether carboxylate compounds have been found to be useful there is needed more economical manufacturing processes whereby such compounds can be economically produced in large volume.
There has been discovered a mixture of polycarboxylic acids or salts thereof, particularly the sodium salts, of 1-hydroxy-3-oxa-1,2,4,5-pentane tetracarboxylic acid (HOPTC) and 3,6-dioxa-1,2,4,5,7,8-octane hexacarboxylic acid (DOOHC) which are highly useful for detergent formulations as a sequesterant or builder. This mixture is prepared by reaction of a combination of maleate and D,L-tartrate salts catalyzed with calcium ion in strongly basic solution.
The synthesis of many ether carboxylates, including the mixture of HOPTC and DOOHC is achieved in an equilibrium reaction wherein starting materials, tartrate and maleate salts, remain in solution at the end of the reaction. In many cases these starting materials are removed only by solvent extraction which is expensive and not ecologically attractive. Large scale production of such ether carboxylates incur large costs for recovery of reactants and an economically and environmentally acceptable means for recovering unreacted starting material is practically a requirement for industrial production of commercial quantities of these ether carboxylates.